In wireless communication system environments, such as cellular radio environments, the conditions under which radio waves propagate typically vary continually, which causes variation as a function of time and place, i.e. fading, in a radio signal. Changes occurring in the impulse response of a channel may be caused either by physical changes in the medium (variation of the refractive index of the medium as a function of temperature, pressure and partial pressure of water vapour, for example) or changes in the geometry of the connection (movement of the transmitter or receiver or obstacle on the connection). Fading can be fast or slow, and usually both types of fading occur on a radio channel simultaneously.
Fast fading of a signal is caused by multipath propagation characteristic of the cellular radio environment, wherein the signal propagates via several different routes between a transmitter and a receiver. Such a channel is called Rayleigh fading channel (comprises only multipath-propagated signal components) or Rice fading channel (a received signal also comprises a stable part, i.e. a directly propagated part or a strongly mirror-reflected part). The different signal components comprising different amplitudes and phases are summed at the receiver and, depending on the phase differences between the signal components, they either amplify or attenuate each other, thus making the level of the sum signal vary considerably, up to dozens of decibels even over as short a distance as half a wavelength. Fast fading is typical particularly of urban areas with many reflecting surfaces, such as walls of tall buildings. FIG. 1 shows an example of a multipath propagation environment of the Rice type, wherein from the output of a subscriber terminal 110, the receiver antenna of a base station 100 receives a directly propagated signal component 106 and signal components 102 and 104 reflected from the walls of buildings 108. Naturally, the number of the multipath-propagated signals can be other than two. Fast fading causes the signal level to abruptly and dramatically drop typically at intervals of half a wavelength, at 17-centimeter intervals within the frequency range of 900 MHz of a GSM system, for example. The signal level can drop even below the dynamic sensitivity level of the receiver, in which case the signal will be lost.
Slow fading of a signal is, in turn, caused by the fact that the propagation path of a radio signal comprises a varying number of different factors causing attenuation, typically natural obstacles or buildings. The effect of slow fading on the propagating signal is at least one grade slower than the variation in the signal level caused by fast fading.
Owing to variations in a radio channel and also to handovers (a moving subscriber terminal moves from one cell to another during a communication connection and changes base stations) typical of the cellular radio system, different equipment in a wireless communication system, such as base stations and subscriber terminals of the radio channel, measure the quality of a received signal at regular intervals with different counting methods, for example by calculating the ratio of the number of erroneous bits to the number of received ones (BER), or by measuring the level of the received signal. In the GSM systems, for example, a base station transmits the measurement information about the signal received from the subscriber terminal to a base station controller, which, on the basis of the received information, can, for example, command the subscriber terminal to perform handover or to raise the transmission power. If the signal received by the base station from the subscriber terminal is weak because the location of the subscriber terminal is disadvantageous to the propagation of the signal, raising the transmission power may not suffice and, owing to the increased level of interference, it can even be harmful to the other users of the network. In addition, it is advantageous to keep the power consumption of a portable terminal as low as possible; therefore, it is advisable to favour other solutions to ensure reception.
In the existing radio communication systems and in those being developed, a great disadvantage is that no information on the quality of a signal received by the network part of the system is transmitted to the transmitter of the message. The only measurement information transmitted to the user of the subscriber terminal is about a signal received, but not transmitted, by the terminal. In mobile telephone systems, for example, on the display of a subscriber terminal there is typically an indicator comprising a line of bars, the number of bars therein indicating the strength of the received signal, i.e. the field strength of the transmitting antenna of the base station, but no information on the quality of the signal transmitted by the subscriber terminal is indicated on the display of the subscriber terminal. If the connection is poor because of a low level of the signal received by the base station receiver, the call may deteriorate at the receiving end such that letters or even entire words are lost of the received speech. In this case, the user of the subscriber terminal does not necessarily know that the quality of the connection has deteriorated since, for example, he or she may receive the speech of the other party well when the power of the antenna of the base station or transmitter is greater than that of the power of the antenna or transmitter of the terminal in his or her use. Nor does the transmitter of a data message receive any information about the reason why the transmission of his or her messages fails or retransmissions take time. Hence, it is not possible for the establisher of the connection to affect the quality of the radio channel he or she uses for example by searching the most advantageous spot in view of reception; in many cases, such a spot could actually be found in the vicinity.